JPH0118969B2 - - Google Patents

Description

[Detailed description of the invention]

This invention provides a low
This invention relates to a method for manufacturing Mn high-strength steel. Conventionally, it has been thought that in Al-killed steel, solid solute N, which is the main cause of aging, becomes AlN and precipitates sufficiently, so that aging does not occur.
Al-killed steel was widely used in various fields without taking measures against strain aging caused by processing. However, in recent years, Al-killed high-strength steel has been developed, which is water-cooled after hot rolling and used with the resulting ferrite-bainite structure without heat treatment. The demand for such high-strength steel is increasing, as it has been put to practical use mainly in thick plate materials for shipbuilding because it has a higher strength than other materials. As a result, information has begun to come in frequently that there are cases where the toughness deteriorates significantly. Therefore, the present inventors conducted research to investigate the cause of the toughness deterioration that occurs in Al-killed steel materials manufactured by rapid cooling after hot rolling, and found that Al
Even for killed steel, if it is rapidly cooled, solid solution C and solid solution N will not completely precipitate during cooling, and will remain in solid solution, especially in the ferrite base, causing strain aging. They came to the conclusion that strain concentrates in the ferrite part, which is softer than bainite, and particularly deteriorates the toughness of that part.
We also found that this strain aging is particularly large in low-Mn materials. From the above-mentioned viewpoint, the present inventors continued research to produce low-Mn high-strength steel that does not suffer from strain aging embrittlement at low cost and with high efficiency. When producing high-strength steel, the N content in the steel is kept low, and quenching is stopped at around 300°C to 250°C.
Preferably, cooling between 300 and 220°C over a period of 5 minutes or more will precipitate sufficient supersaturated N and C, making it non-aging like normal Al-killed steel and the same as conventional rapidly cooled steel. Furthermore, the bond toughness during high heat input welding of steel in the low Mn range can be significantly improved by reducing the N content of the steel. It was also confirmed. This improvement in bond toughness during high heat input welding is particularly desirable for shipbuilding plates where high heat input welding is frequently performed. This invention was made based on the above findings, and includes: C: 0.1 to 0.2% (hereinafter, % is a weight percentage), Si: 0.5% or less, Mn: 0.6 to 1.1%, sol.Al: 0.01 〜0.08%, N: 0.0030% or less, V: 0.08% or less, Ti: 0.08% or less, Nb: 0.08% or less, Contains one or more of the following, Fe and unavoidable impurities: the remainder, After heating the steel with a composition of 900 to 1200℃,
Finishing temperature: Hot rolling at 900-750℃, followed by
Perform rapid cooling between 800 and 500℃ at an average cooling rate of 10 to 60℃/sec until the temperature reaches 300℃, and then
By cooling the temperature between ~100℃ over 5 minutes and lowering the temperature to room temperature, we can produce high-strength steel that does not suffer from strain aging embrittlement and has excellent bond toughness during high heat input welding. It is characterized in that it can be manufactured efficiently and reliably. Next, in the method for producing high-strength steel of the present invention, the reason why the composition of the target steel and the heating, rolling, and cooling conditions are limited as described above will be explained. (A) Ingredient composition (a) C The C component has the effect of ensuring the strength of steel, and is included in a predetermined amount in relation to the amount of Mn, which has the same effect, but the content is 0.1 If the Mn content is less than 0.2%, sufficient strength of the steel in the low Mn region cannot be ensured, while if the Mn content exceeds 0.2%, the increase in strength will be excessive and the toughness of the base material will deteriorate. The content was set at 0.1-0.2%. (b) Si The Si component is used for deoxidizing steel, adjusting base material strength, etc.
It is added for various purposes, but 0.50
If the content exceeds 0.5%, the toughness of the base material will deteriorate, so the content was set at 0.5% or less. (c) Mn The Mn component has the effect of increasing the strength of the base material and improving its toughness, but its content is
If it is less than 0.6%, the desired effect cannot be obtained. On the other hand, steel with a Mn content of more than 1.1% has low aging properties, and even if the treatment of the present invention is applied, it will not be possible to obtain an effect commensurate with the cost.
%. Figure 1 shows C: 0.17%, Si: 0.25%, Nb:
After rolling steel containing 0.012%, sol.Al: 0.043%,
3% strain aging effect and Mn when water cooled to room temperature
This is a diagram showing the relationship with the Mn content, and from Figure 1 it can be seen that for steel with a Mn content of 1.1% or less, the Mn content is 3%.
It is clear that the amount of embrittlement after strain aging is large. The strain aging effect is calculated by the difference between the fracture surface transition temperature in the Charpy impact test before strain aging and the fracture surface transition temperature after strain aging, that is, ΔvTs = vTs (before strain aging) - vTs (after strain aging). It was expressed as ΔvTs. (d) sol.Al The sol.Al component is used as a deoxidizing agent and also has the effect of fixing N in steel as AlN, but if its content is less than 0.01%, the N The desired fixing effect cannot be obtained, and on the other hand, if the content exceeds 0.08%, the cleanliness of the steel will decrease, so the content was set at 0.01 to 0.08%. (e) N If N exceeds 0.003%, the first
As is clear from the figure, the strain aging properties of the base metal increase and at the same time the toughness of the weld bond deteriorates, so the content was set at 0.003% or less. Figure 2 is a diagram examining the effect of N content on high heat input welding bond toughness for the same steel as in Figure 1.
It can be seen that by setting the content to 0.003% or less, the weld bond toughness becomes extremely good. Note that high Mn steel exhibits good weld bond toughness even if the N content increases to a certain extent, but this results in a disadvantage due to increased cost. Therefore, from Figures 1 and 2, it is clear that low Mn
- It can be seen that by using low N steel, a steel material with good base material performance and bond toughness and at low cost can be obtained. (f) V, Ti, and Nb These components each have a carbide- or nitride-forming effect, and fix solid solution C and N in steel to improve the strength of the base metal, but each of them accounts for 0.08%. If the content exceeds 0.08%, there is a tendency to deteriorate the toughness of the base metal, and it also has a negative effect on the weld bond toughness, so the content of each element was set at 0.08% or less. For V, the content must be limited to 0.075% or less, and for Ti and Nb, it is 0.07% or less and 0.04%, respectively.
It is more preferable to limit it to the following. (B) Heating temperature If the heating temperature is less than 900°C, the γ-hardening of the steel and the solid solution uniformity of carbides will be insufficient, and on the other hand, even if the heating temperature exceeds 1200°C, there will be no change in the γ-hardening effect. Otherwise, the γ grains become too large and deteriorate the toughness of the base material, so the temperature is
The temperature was set at 900-1200℃. (C) Finishing temperature of rolling If the finishing temperature exceeds 900°C, the effect of processing is small and no improvement in toughness can be expected through processing.On the other hand, if rolling is finished at a temperature of less than 750°C, processing If it is too strong, the rolling effect of α + γ2 phase region will appear, which will deteriorate the toughness, so the finishing temperature of rolling was set to 900.
The temperature was set at ~750℃. (D) Average cooling rate between 800 and 500℃ If the average cooling rate between 800 and 500℃ is less than 10℃/sec, the desired strength of the steel material cannot be secured; If the cooling rate exceeds this, the base material will harden too much and the toughness will deteriorate, so the cooling rate was set at 10 to 60°C/sec. Figure 3 shows C: 0.17%, Si: 0.25%, Mn:
For steel containing 0.63%, sol.Al: 0.043%, Nb: 0.012%, water cooling to 300℃ after hot rolling,
This is a diagram showing the relationship between the average cooling rate between 800 and 500°C and the mechanical properties of a steel material that was cooled between 300 and 220°C for 8 minutes to promote precipitation. It is clear that good results can be obtained with an average cooling rate in the range of 10 to 60°C/sec. (E) Rapid cooling stop temperature If the rapid cooling stop temperature of water cooling etc. is higher than 300℃,
Since the strengthening effect of the steel is small and it is difficult to secure the desired strength, it was decided to rapidly cool the steel until the temperature was 300°C or less. of course,
Rapid cooling can be done up to 300°C, and care should be taken because if the temperature is much lower than that, it will be difficult to fix solid solution C and solid solution N in the next step. (F) Cooling time from 300 to 100°C After rapid cooling following hot rolling, cooling from 300 to 100°C for 5 minutes or more reduces solid solution C and solid solution N.
This is done to precipitate the
If slow cooling is carried out from a temperature exceeding 100°C, precipitation will occur easily, but the strength of the steel will be greatly reduced. On the other hand, if the temperature is below 100°C, precipitation will not occur sufficiently. If the temperature is not kept within this temperature range for 5 minutes or more, sufficient precipitation will not occur and strain aging will occur, so
It was decided that the temperature should be cooled for at least 5 minutes. Note that the slow cooling temperature range is preferably 300 to 220°C. This is because the degree of precipitation decreases rapidly below 220°C, and near 200°C is a temperature range of so-called blue brittleness, where N-induced precipitation aging embrittlement is significant. Therefore, it is recommended to precipitate a large amount of solid solution N before the temperature drops to around 200°C to prevent precipitation aging embrittlement caused by N. Figure 4 shows the relationship between the 3% strain aging effect and the precipitation treatment temperature of solid solute C and N when water-cooled to room temperature after hot rolling for the same steel as in Figure 3. This is a diagram showing the relationship between temperature and ΔvTs when heated and held for 10 minutes.
It can be seen that treatment between 300 and 220°C is particularly effective in preventing aging. In addition, with high N steel, sufficient effects cannot be obtained even after holding for about 10 minutes;
It is expected that the process will take more than 30 minutes. Figure 5 shows that similar steel was hot-rolled and water-cooled to 300°C, but after 3% strain aging.
It is a diagram showing the influence of gradual cooling (aging treatment) between 100°C, and it is clear that in the case of continuous cooling, it is necessary to cool for 5 minutes or more. Next, the present invention will be explained with reference to examples and in comparison with comparative steel. Example First, steels A to L having the chemical compositions shown in Table 1 are melted by a melting method that combines a converter steelmaking method and a vacuum degassing method (RH method), and then N is melted in the air.
A slab with a thickness of 150 mm was obtained by performing continuous casting while constantly shielding with an inert gas such as Ar to prevent absorption of gas. Subsequently, each of these was heated to the temperatures shown in Table 2, hot-rolled under the conditions also shown in Table 2 to a finished plate thickness of 30 to 20 mm, and immediately water-cooled to 300°C. I went (However, test number 13
and 14). At this time, the average cooling rate between 800 and 500°C was within the range of 30 to 60°C/sec, although there were some differences depending on the plate thickness. In addition,
The individual results are as shown in Table 2. Following this, test numbers 1 to 12 were slowly cooled in the temperature range of 300 to 100°C for 20 minutes (however, approximately 7 minutes in the range of 300 to 220°C), and then allowed to cool in the atmosphere. Test numbers 13 and 14 are at 450℃ and 100℃, respectively.
After cooling with water until

【table】

[Table] Allowed to cool in the atmosphere. The mechanical properties of the steel materials thus obtained were investigated, and the results are also shown in Table 2. In addition, welding uses the single-sided SAW method, heat input: 170,000 to 230,000 J/
I went under cm. As is clear from the results shown in Table 2, the high tensile strength steels obtained by methods 1 to 7 of the present invention have a
The ΔvTs of % strain aging is small at 20℃ or less, the tensile strength is 50Kgf/mm ² or more, and the vTs is -40℃ or less, which are excellent properties. The absorption value (vE _-20 ) is high at over 7 Kg-m, whereas the steel obtained by the comparison method Test No. 8 has a large ΔvTs due to high N; shows good performance for high Mn steel but about 20 compared to low Mn steel.
% cost increase, test number 10
The one according to Test No. 11 has a low C content and has a tensile strength of less than 50Kgf/mm ² .
It can be seen that the material according to test number 12 had poor toughness because the C content was too high. As described above, according to the present invention, a low-cost, low-cost product that has high manufacturing efficiency and does not cause strain aging embrittlement.
Mn high-strength steel can be easily obtained, and the safety of welded structures can be further improved, resulting in industrially useful effects.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between 3% strain aging effect and Mn content, Figure 2 is a diagram showing the influence of N content on high heat input welding bond toughness, and Figure 3 is a diagram showing the relationship between 3% strain aging effect and Mn content.
A diagram showing the relationship between the average cooling rate at 500℃ and mechanical properties. Figure 4 shows the 3% strain aging effect and solid solution C, N.
A diagram showing the relationship between precipitation treatment temperature, Figure 5 is 3%
FIG. 3 is a diagram showing the influence of cooling time between 300 and 100° C. on strain aging.

Claims (1)

[Claims] 1 Contains C: 0.1 to 0.2%, Si: 0.5% or less, Mn: 0.6 to 1.1%, sol.Al: 0.01 to 0.08%, N: 0.0030% or less, and further contains V: 0.08% or less, Ti: 0.08% or less, Nb: 0.08% or less, containing one or more of the following, Fe and unavoidable impurities: the remainder, the steel having a composition (more than 900% by weight) of
After heating to 1200℃, hot rolling is performed at a finishing temperature of 900 to 750℃, followed by an average cooling rate of 800 to 500℃.
A high-strength steel characterized by rapidly cooling at a rate of 10 to 60°C/sec until the temperature reaches 300°C or less, and then cooling from 300 to 100°C over 5 minutes to bring the temperature down to room temperature. manufacturing method.